Automatic control system for the control of physical or chemical processes have been known for a long time and are further refined particularly by using digital or analog computer equipment. One control system that has considerable advantages in many applications involves so-called PI or PID controllers. These controllers generate a control signal composed of a first portion that is proportional to (P), a second portion that is the time integral of (I) and optionally a third portion that is the time derivative of (D) the deviation of a process variable signal from a setpoint.
In cases where these PI or PID controllers are used in connection with a circuitry that limits the size of the control signal or in connection with a circuitry that selects among two or more of the controllers described, the problem arises that the controller which is not actually used does not have a "feedback" through the controlled process. Therefore the integrator portion of the controller winds up until the controller has reached the maximum output signal possible. This is very undesirable because the control signal of the respective windup controller then is not representative of the desired control action anymore when the respective controller becomes active again.